Field of the Disclosure
Embodiments of the present disclosure relate to an organic light emitting display device, and more particularly, to an organic light emitting display device which is capable of rapidly sensing a characteristic variation in a pixel including an organic light emitting diode and a driving transistor.
Discussion of the Related Art
An organic light emitting display device includes an organic light emitting layer which emits light by recombination of hole and electron, whereby the organic light emitting display device emits light in itself. Also, since the organic light emitting display device emits light in itself, there is no problem related with a viewing angle. In addition, the organic light emitting display device has advantages of rapid response speed and low power consumption. In this respect, the organic light emitting display device has been attracted as a next-generation flat panel display.
The organic light emitting display device may include a plurality of pixels for displaying images. Each pixel may include an organic light emitting diode having an organic light emitting layer between anode and cathode electrodes, and a pixel circuit for making the organic light emitting diode emit light. The pixel circuit may include a switching transistor, a driving transistor, and a capacitor. As the switching transistor is driven (e.g., switched) by a gate signal, the switching transistor supplies a data voltage to the driving transistor. As the driving transistor is driven (e.g., switched) by the data voltage supplied from the switching transistor, the driving transistor controls a current flowing to the organic light emitting diode, and also controls a light emission of the organic light emitting diode. The capacitor stores charge responsive to a voltage between gate and source terminals of the driving transistor, and drives (e.g., switches) the driving transistor by the use of stored voltage. The organic light emitting diode emits light by the current supplied from the driving transistor.
In the organic light emitting display device according to the related art, a characteristic variation of the driving transistor such as variations in mobility and threshold voltage (Vth) of the driving transistor may occur in each pixel due to a manufacturing deviation, whereby an amount of current for driving the organic light emitting diode may vary, and thus a luminance deviation may occur between each of pixels. In order to overcome this problem, the Unexamined Publication Number P10-2013-0066449 in the Korean Intellectual Property Office (hereinafter, referred to as ‘prior art document’) discloses an external compensation technique for compensating the characteristic variation of pixel by sensing the characteristic variation of pixel and reflecting the sensing result on data of the pixel.
In the above-mentioned prior art document, as shown in FIGS. 1 and 2, a data line connected with each pixel (P) is used as a sensing line 11, the sensing line 11 is charged with the current flowing in the driving transistor of the pixel (P), a voltage (Vout) charged in the sensing line 11 is sensed by an analog-to-digital converter (ADC), and the current flowing in the driving transistor of the pixel (P) is analogized (e.g., indirectly estimated) based on the sensed voltage. That is, in case of the above-mentioned prior art document, the voltage is sensed by the analog-to-digital converter (ADC) of voltage sensing method without measuring the actual current, and then the current flowing in the driving transistor is analogized based on the sensed voltage. In other words, the sensed voltage is used as a proxy for the current through the driving transistor.
However, in the above-mentioned prior art document, a sensing time (Tsen) for the sensing line 11 is increased due to large parasitic resistance (Rp) and large parasitic capacitance (Cp) of the sensing line 11; a sensing time (Tsen) for sensing a small current value corresponding to a low grayscale value is especially prolonged or increased. Also, the parasitic resistance (Rp) and parasitic capacitance (Cp) vary depending on a position of the sensing line 11, thereby causing errors in the sensing voltage. In case of the above-mentioned prior art document, since the data line, which is connected with both the organic light emitting diode and a source electrode of the driving transistor, is also used as the sensing line 11, undesired emissions of the organic light emitting diode occur in the low grayscale, which results in lowering of contrast ratio due to the increased luminance of low grayscale.